Field of the Invention
This invention relates to voltage controlled band-pass filters and in particular it relates to the use of such filters in radio transmitter/receivers.
Description of the Related Art
A reduction in cost and an improvement in reliability are amongst the benefits that may be obtained when it is possible to achieve a reduction in the number of separate components of a radio transmitter/receiver. Improvements in the performance of the radio may also be obtained by means of the integration of components previously required to be provided separately. Also a minimum bandwidth for the IF filter, consistent with adequate reception of the wanted signal will enhance receiver performance considerably.
It is an object of the present invention to provide an improved means for frequency and channel selection for both receiver and transmitter in a radio transmitter/receiver while reducing the number of separate components required.
According to the invention there is provided a voltage controlled band-pass filter in a phase locked loop with the output from a phase detector applied through a loop filter as a control voltage, the output from the phase detector being proportional to the difference in phase of first and second signals input to the phase detector with the first signal fed directly to the phase detector from a local oscillator and the second signal fed from said local oscillator to the phase detector through the voltage controlled band-pass filter.